REST API: Introduce themes endpoint to expose theme-supports values for the active theme.

In order to correctly render parts of its UI, the new editor needs to be aware of the active theme's post-formats and post-thumbnails support. This data is exposed by querying for the active theme on a new /wp/v2/themes endpoint for sufficiently privileged users.

Merges [43734], [43735] to trunk.

props desrosj.
Fixes #45016.

Built from https://develop.svn.wordpress.org/trunk@43985


git-svn-id: https://core.svn.wordpress.org/trunk@43817 1a063a9b-81f0-0310-95a4-ce76da25c4cd